Build _testmultiphase.c with doc strings

Author: fangerer

graalpython/com.oracle.graal.python.cext/modules/_testmultiphase.c

@@ -6,6 +6,9 @@
 /* Testing module for multi-phase initialization of extension modules (PEP 489)
  */
 
+/* For the tests, we need doc strings. */
+#define WITH_DOC_STRINGS 1
+
 #include "Python.h"
 
 /* Example objects */